The census found a previously unknown population
A census of critically endangered western lowland gorillas has shown that populations are doing much better than anyone expected.
The census found 125,000 of the apes alive and well in two adjacent areas in the northern part of the Republic of Congo covering 47,000 sq km.
A census in the 1980s estimated that about 100,000 remained, but numbers were since thought to have halved.
Hunting and the ebola virus was thought to have slashed population numbers.
The census data were released by the Wildlife Conservation Society (WCS) during the International Primatological Society Congress in Edinburgh, UK.
In the latest census, researchers combed rainforests and isolated swamps to count gorilla "nests", to accurately estimate the population.
Gorillas build nests each night from leaves and branches for sleeping.
Population densities ranged as high as eight individuals per square kilometre in one particularly rich forest patch, which ranks among the highest gorilla densities ever recorded.
WCS said a combination of factors was responsible for such high numbers of gorillas.
These included successful long-term conservation management of the Republic of Congo's protected areas, remoteness and inaccessibility of some of the key locations where the gorillas were found, and a food-rich habitat.
In all, the researchers estimated a total of 125,000 gorillas in just this northern Congo area.
A total of 73,000 came from the Ntokou-Pikounda region and another 52,000 from the Ndoki-Likouala area - including a previously unknown population of nearly 6,000 gorillas living in an isolated swamp.
WCS warned that many of the gorillas live outside of existing protected areas, though the Government of Congo has committed to creating a new national park in the Ntokou-Pikounda region.
"We knew from our own observations that there were a lot of gorillas out there, but we had no idea there were so many," said Dr Emma Stokes, who led the survey efforts in Ndoki-Likouala.
"We hope that the results of this survey will allow us to work with the Congolese government to establish and protect the new Ntokou-Pikounda protected area."
Jillian Miller, executive director of conservation group The Gorilla Organization commented: "The discovery of such a large population of western lowland gorillas is absolutely fantastic news for the sub-species and for conservationists, but we should be careful not to be too complacent.
"The area where these gorillas have been found is in the path of the Ebola virus, which has wiped out large numbers of Western lowland gorillas during the past 25 years."
She added: "Numbers are less important than trends, and sadly the trend for all gorilla sub-species, apart from the mountain gorillas, has been a downturn in population figures."
The two principal threats to gorillas come from hunting for bushmeat and the spread of the Ebola virus, which is lethal to gorillas as well as humans.
Western lowland gorillas are one of four recognized gorilla sub-species, which also include mountain gorillas, eastern lowland gorillas, and Cross River gorillas.
All are classified as "critically endangered" by the IUCN, except eastern lowland gorillas, which are endangered.